Patent Document 1 discloses one example of a fastening structure of a resin component that fastens a metal component and a resin component to each other. In the fastening structure disclosed in Patent Document 1, a box portion is formed on a back surface of a resin component, which is a side cover in this publication. A retainer plate to which a nut is secured is attached to the box portion. An engaging piece and a retaining protrusion are formed on the retainer plate. When the retainer plate is inserted into the box portion, the retainer plate is secured to the box portion by the engaging piece and the retaining protrusion. In this state, a fastening bolt is screwed into the nut of the retainer plate via a through hole of the metal component, which is a vehicle body panel in this publication, so that the side cover is fastened to the vehicle body panel.